Expression of vinexin alpha in the dorsal half of the eye and in the cardiac outflow tract and atrioventricular canal

Midbody localization of vinexin recruits rhotekin to facilitate cytokinetic abscission

Vinexin is a SH3 domain-containing adaptor protein that has diverse roles in cell adhesion, signal transduction, gene regulation and stress granule assembly. In this study, we found that vinexin localizes at the midbody during cell division and facilitates cytokinesis. Knockdown of vinexin in HeLa cells delayed the mitotic cell cycle progression and increased the time of cell abscission and the failure to resolve the cytoplasmic bridge. Midbody-localized vinexin is essential for recruiting rhotekin to this structure for cytokinesis because overexpression of a vinexin mutant without a rhotekin-binding motif or knockdown of rhotekin also impaired cytokinetic abscission and increased the number of cells arrested at the midbody stage. Aberrant expression of vinexin and rhotekin in various cancers has been implicated to promote metastasis because of their functions in cell adhesion and signaling. Our findings reveal a novel role of vinexin and rhotekin in cytokinetic abscission and provide another perspective of how both molecules may affect oncogenic transformation via this fundamental cell cycle process.

Arsenite-activated JNK signaling enhances CPEB4-Vinexin interaction to facilitate stress granule assembly and cell survival

Stress granules (SGs) are compartmentalized messenger ribonucleoprotein particles (mRNPs) where translationally repressed mRNAs are stored when cells encounter environmental stress. Cytoplasmic polyadenylation element-binding protein (CPEB)4 is a sequence-specific RNA-binding protein and translational regulator. In keeping with the results obtained from the study of other RNA-binding proteins, we found CPEB4 localized in SGs in various arsenite-treated cells. In this study, we identified that Vinexin, a CPEB4-interacting protein, is a novel component of SGs. Vinexin is a SH3-domain-containing adaptor protein and affects cell migration through its association with Vinculin to localize at focal adhesions (FAs). Unexpectedly, Vinexin is translocated from FAs to SGs under arsenite-induced stress. The recruitment of Vinexin to SGs depends on its interaction with CPEB4 and influences SG formation and cell survival. Arsenite-activated c-Jun N-terminal kinase (JNK) signaling enhances the association between CPEB4 and Vinexin, which consequently facilitates SG localization of Vinexin. Taken together, this study uncovers a novel interaction between a translational regulator and an adaptor protein to influence SG assembly and cell survival.

Cdk5 regulates multiple cellular events in neural development, function and disease

Cyclin-dependent kinases (CDKs) generally regulate cell proliferation in dividing cells, including neural progenitors. In contrast, an unconventional CDK, Cdk5, is predominantly activated in post-mitotic cells, and involved in various cellular events, such as microtubule and actin cytoskeletal organization, cell-cell and cell-extracellular matrix adhesions, and membrane trafficking. Interestingly, recent studies have indicated that Cdk5 is associated with several cell cycle-related proteins, Cyclin-E and p27(kip1) . Taking advantage of multiple functionality, Cdk5 plays important roles in neuronal migration, layer formation, axon elongation and dendrite arborization in many regions of the developing brain, including cerebral cortex and cerebellum. Cdk5 is also required for neurogenesis at least in the cerebral cortex. Furthermore, Cdk5 is reported to control neurotransmitter release at presynaptic sites, endocytosis of the NMDA receptor at postsynaptic sites and dendritic spine remodeling, and thereby regulate synaptic plasticity and memory formation and extinction. In addition to these physiological roles in brain development and function, Cdk5 is associated with many neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. In this review, I will introduce the physiological and pathological roles of Cdk5 in mammalian brains from the viewpoint of not only in vivo phenotypes but also its molecular and cellular functions.

Modulation of dorsal root ganglion development by ErbB signaling and the scaffold protein Sorbs3

The multipotent cells of the vertebrate neural crest (NC) arise at the dorsal aspect of the neural tube, then migrate throughout the developing embryo and differentiate into diverse cell types, including the sensory neurons and glia of the dorsal root ganglia (DRG). As multiple cell types are derived from this lineage, it is ideal for examining mechanisms of fate restriction during development. We have isolated a mutant, ouchless, that specifically fails to develop DRG neurons, although other NC derivatives develop normally. This mutation affects the expression of Sorbs3, a scaffold protein known to interact with proteins involved in focal adhesions and several signaling pathways. ouchless mutants share some phenotypic similarities with mutants in ErbB receptors, EGFR homologs that are implicated in diverse developmental processes and associated with several cancers; and ouchless interacts genetically with an allele of erbb3 in DRG neurogenesis. However, the defect in ouchless DRG neurogenesis is distinct from ErbB loss of function in that it is not associated with a loss of glia. Both ouchless and neurogenin1 heterozygous fish are sensitized to the effects of ErbB chemical inhibitors, which block the development of DRG in a dose-dependent manner. Inhibitors of MEK show similar effects on DRG neurogenesis. We propose a model in which Sorbs3 helps to integrate ErbB signals to promote DRG neurogenesis through the activation of MAPK and upregulation of neurogenin1.

Cell adhesion and its endocytic regulation in cell migration during neural development and cancer metastasis

Cell migration is a crucial event for tissue organization during development, and its dysregulation leads to several diseases, including cancer. Cells exhibit various types of migration, such as single mesenchymal or amoeboid migration, collective migration and scaffold cell-dependent migration. The migration properties are partly dictated by cell adhesion and its endocytic regulation. While an epithelial-mesenchymal transition (EMT)-mediated mesenchymal cell migration requires the endocytic recycling of integrin-mediated adhesions after the disruption of cell-cell adhesions, an amoeboid migration is not dependent on any adhesions to extracellular matrix (ECM) or neighboring cells. In contrast, a collective migration is mediated by both cell-cell and cell-ECM adhesions, and a scaffold cell-dependent migration is regulated by the endocytosis and recycling of cell-cell adhesion molecules. Although some invasive carcinoma cells exhibit an EMT-mediated mesenchymal or amoeboid migration, other cancer cells are known to maintain cadherin-based cell-cell adhesions and epithelial morphology during metastasis. On the other hand, a scaffold cell-dependent migration is mainly utilized by migrating neurons in normal developing brains. This review will summarize the structures of cell adhesions, including adherens junctions and focal adhesions, and discuss the regulatory mechanisms for the dynamic behavior of cell adhesions by endocytic pathways in cell migration in physiological and pathological conditions, focusing particularly on neural development and cancer metastasis.

Crucial role of vinexin for keratinocyte migration in vitro and epidermal wound healing in vivo

In the process of tissue injury and repair, epithelial cells rapidly migrate and form epithelial sheets. Vinexin is a cytoplasmic molecule of the integrin-containing cell adhesion complex localized at focal contacts in vitro. Here, we investigated the roles of vinexin in keratinocyte migration in vitro and wound healing in vivo. Vinexin knockdown using siRNA delayed migration of both HaCaT human keratinocytes and A431 epidermoid carcinoma cells in scratch assay but did not affect cell proliferation. Induction of cell migration by scratching the confluent monolayer culture of these cells activated both EGFR and ERK, and their inhibitors AG1478 and U0126 substantially suppressed scratch-induced keratinocyte migration. Vinexin knockdown in these cells inhibited the scratch-induced activation of EGFR, but not that of ERK, suggesting that vinexin promotes cell migration via activation of EGFR. We further generated vinexin (-/-) mice and isolated their keratinocytes. They similarly showed slow migration in scratch assay. Furthermore, vinexin (-/-) mice exhibited a delay in cutaneous wound healing in both the back skin and tail without affecting the proliferation of keratinocytes. Together, these results strongly suggest a crucial role of vinexin in keratinocyte migration in vitro and cutaneous wound healing in vivo.

Changes in Vinexin Expression Patterns in the Mouse Testis Induced by Developmental Exposure to 17Beta-Estradiol1

In the seminiferous epithelium, numerous cell interactions between Sertoli cells and Sertoli-germ cells are established by specialized proteins so as to maintain the functionality of the testis. Exogenous estrogen exposure can result in alterations to these interactions and cause pathologies, including impaired spermatogenesis and tumorigenesis. In the present study, with the aim of finding markers of the action of estrogenic compounds in the mammalian testis, we focused on investigating molecules that are linked to cellular junctions. We found that the testicular vinexin (sorbin and SH3 domain-containing protein 3, encoded by the Sorbs3 gene) pattern underwent significant changes after developmental exposure to 17beta-estradiol (E(2)). Vinexin is an adaptor protein that is implicated in cell adhesion and actin-cytoskeletal reorganization. We characterized, at the protein and mRNA levels, the expression patterns of vinexin isoforms during testis development and in defined cell types from the seminiferous tubule. The protein expression patterns of vinexin-interacting proteins flotillin 1 and vinculin were also analyzed. Thus, we have identified a novel association between a vinexin isoform and germ cells, which contrasts with the predominant localization of the gamma isoform in Sertoli cells. The effects of E(2) on the testes of developmentally exposed mice were evident, with total depletion of the germ-cell-associated vinexin isoform and a noticeable decrease in Sertoli-cell-related vinexin gamma.

Solution structure of the first SH3 domain of human vinexin and its interaction with vinculin peptides

Solution structure of the first Src homology (SH) 3 domain of human vinexin (V_SH3_1) was determined using nuclear magnetic resonance (NMR) method and revealed that it was a canonical SH3 domain, which has a typical beta-beta-beta-beta-alpha-beta fold. Using chemical shift perturbation and surface plasmon resonance experiments, we studied the binding properties of the SH3 domain with two different peptides from vinculin hinge regions: P856 and P868. The observations illustrated slightly different affinities of the two peptides binding to V_SH3_1. The interaction between P868 and V_SH3_1 belonged to intermediate exchange with a modest binding affinity, while the interaction between P856 and V_SH3_1 had a low binding affinity. The structure and ligand-binding interface of V_SH3_1 provide a structural basis for the further functional study of this important molecule.

Involvement of phosphatases in the anchorage-dependent regulation of ERK2 activation

Activation of extracellular signal-regulated kinase (ERK) is known to be regulated by cell adhesion, namely "anchorage dependence". Most studies on the anchorage-dependent regulation have focused on the upstream activating components. We previously reported that the focal adhesion protein vinexin beta can induce the anchorage-independent activation of ERK2. We show here that vinexin beta-induced anchorage-independent activation of ERK2 involves prevention of the dephosphorylation of ERK2, but not the promotion of MEK1 or Raf1 activity. Furthermore, knockdown of vinexin beta resulted in a faster dephosphorylation of ERK2 in A549 cells. Moreover, the coexpression of MKP3/rVH6, an ERK2 specific phosphatase, suppressed the anchorage-independent activation of ERK2 induced by vinexin beta. These results suggest that vinexin beta can prevent the dephosphorylation of ERK2 stimulated by cell detachment, leading to the anchorage-independent activation of ERK2. Furthermore, we found that phosphatase activity directed against activated ERK2 was higher in suspended cells than in adherent cells. In addition, orthovanadate efficiently induces anchorage-independent activation of ERK2 without marked activation of MEK1 in NIH3T3 cells. These observations suggest that the anchorage dependence of ERK1/2 activation is regulated not only by upstream kinases, Raf1 and MEK, but also by phosphatases acting against ERK1/2 and that vinexin beta can induce anchorage-independent activation of ERK by preventing the inactivation of ERK1/2.

Vinexin β regulates the phosphorylation of epidermal growth factor receptor on the cell surface

Epidermal growth factor (EGF) regulates various cellular events, including proliferation, differentiation, migration and oncogenesis. In this study, we found that exogenous expression of vinexin beta enhanced the phosphorylation of 180-kDa proteins in an EGF-dependent manner in Cos-7 cells. Western blot analysis using phospho-specific antibodies against EGFR identified EGFR as a phosphorylated 180-kDa protein. Vinexin beta did not stimulate the phosphorylation of EGFR but suppressed the dephosphorylation, resulting in a sustained phosphorylation. Mutational analyses revealed that both the first and third SH3 domains were required for a sustained phosphorylation of EGFR. Small interfering RNA-mediated knockdown of vinexin beta reduced the phosphorylation of EGFR on the cell surface in HeLa cells. The sustained phosphorylation of EGFR induced by vinexin beta was completely abolished by adding the EGFR-specific inhibitor AG1478 even after EGF stimulation, suggesting that the kinase activity of EGFR is required for the sustained phosphorylation induced by vinexin beta. We also found that E3 ubiquitin ligase c-Cbl is a binding partner of vinexin beta through the third SH3 domain. Expression of wild-type vinexin beta but not a mutant containing a mutation in the third SH3 domain decreased the cytosolic pool of c-Cbl and increased the amount of membrane-associated c-Cbl. Furthermore, over-expression of c-Cbl suppressed the sustained phosphorylation of EGFR induced by vinexin beta. These results suggest that vinexin beta plays a role in maintaining the phosphorylation of EGFR on the plasma membrane through the regulation of c-Cbl.

Abl kinase interacts with and phosphorylates vinexin

Non-receptor tyrosine kinase Abl is a well known regulator of the actin-cytoskeleton, including the formation of stress fibers and membrane ruffles. Vinexin is an adapter protein consisting of three SH3 domains, and involved in signal transduction and the reorganization of actin cytoskeleton. In this study, we found that vinexin alpha as well as beta interacts with c-Abl mainly through the third SH3 domain, and that vinexin and c-Abl were colocalized at membrane ruffles in rat astrocytes. This interaction was reduced by latrunculin B, suggesting an F-actin-mediated regulatory mechanism. We also found that vinexin alpha but not beta was phosphorylated at tyrosine residue when c-Abl or v-Abl was co-expressed. A mutational analysis identified tyrosine 127 on vinexin alpha as a major site of phosphorylation by c- or v-Abl. These results suggest that vinexin alpha is a novel substrate for Abl.

Role of interaction with vinculin in recruitment of vinexins to focal adhesions

Although vinexin was originally identified as a protein binding to the proline-rich hinge region of vinculin, the functions and biochemical properties of the vinexin-vinculin interaction are not known. Here, we determined the affinity of the vinexin-vinculin interaction using surface plasmon resonance measurements and found that vinexin beta interacts with the C-terminal half of vinculin, which mimics an activated "open" form, with a threefold higher affinity than with the full-length "closed" vinculin. Coimmunoprecipitation experiments showed that cell adhesion on fibronectin enhances the vinexin-vinculin interaction. We also show that the interaction with vinculin is necessary for the efficient localization of vinexin alpha and beta at focal adhesions. These observations suggest a model that "activated" vinculin localized at focal adhesions recruits vinexins to focal adhesions.

A novel isoform of Vinexin, Vinexin gamma, regulates Sox9 gene expression through activation of MAPK cascade in mouse fetal gonad

Recent loss-of-function and gain-of-function studies have revealed that transcription factor Sox9 is required for testis formation by governing Sertoli cell differentiation, and thereafter regulating transcription of Sertoli marker genes. In the present study, we identified a novel isoform of Vinexin, which is expressed in somatic cells but not germ cells of sexually indifferent stages of fetal gonads. After the sex is determined, the expression continues in testicular Sertoli cells. Immunohistochemical analyses with a specific antibody to Vinexin indicated that Vinexin gamma is localized in the cytoplasm. Functional studies with C3H10T1/2 cells showed that Vinexin gamma acted as a scaffold protein to activate MEK and ERK through interaction with c-Raf and ERK. Ultimately, Sox9 transcription was induced by Vinexin gamma. This up-regulation of Sox9 expression disappeared when the cells were treated with a specific MEK inhibitor, U0126. To determine the role of Vinexin gamma during gonad formation, the gene was disrupted by targeted mutagenesis. The phenotype displayed by the mice indicated that ERK activation was decreased in the Vinexin gamma(-/-) XY gonads, and Sox9 expression was down-regulated. Thus, Vinexin gamma seems to be implicated in regulation of Sox9 gene expression by modulating MAPK cascade in mouse fetal gonads.

Tbx20 is essential for cardiac chamber differentiation and repression of Tbx2

Tbx20, a member of the T-box family of transcriptional regulators, shows evolutionary conserved expression in the developing heart. In the mouse, Tbx20 is expressed in the cardiac crescent, then in the endocardium and myocardium of the linear and looped heart tube before it is restricted to the atrioventricular canal and outflow tract in the multi-chambered heart. Here, we show that Tbx20 is required for progression from the linear heart tube to a multi-chambered heart. Mice carrying a targeted mutation of Tbx20 show early embryonic lethality due to hemodynamic failure. A linear heart tube with normal anteroposterior patterning is established in the mutant. The tube does not elongate, indicating a defect in recruitment of mesenchyme from the secondary heart field, even though markers of the secondary heart field are not affected. Furthermore, dorsoventral patterning of the tube, formation of working myocardium, looping, and further differentiation and morphogenesis fail. Instead, Tbx2, Bmp2 and vinexin alpha (Sh3d4), genes normally restricted to regions of primary myocardium and lining endocardium, are ectopically expressed in the linear heart tube of Tbx20 mutant embryos. Because Tbx2 is both necessary and sufficient to repress chamber differentiation (Christoffels et al., 2004a; Harrelson et al., 2004), Tbx20 may ensure progression to a multi-chambered heart by repressing Tbx2 in the myocardial precursor cells of the linear heart tube destined to form the chambers.

The Focal Adhesion Protein Vinexin α Regulates the Phosphorylation and Activity of Estrogen Receptor α

Steroid receptors are transcription factors that regulate hormone-responsive genes and whose activity is controlled by their interaction with numerous other proteins. Observations reported here reveal that estrogen receptors alpha and beta (ERalpha and ERbeta), androgen receptor, and glucocorticoid receptor bind in vitro to vinexin alpha, a multiple SH3 motif-containing protein associated with the cytoskeleton. The SH3 domains are not involved in this interaction. Furthermore, we demonstrate that vinexin alpha stimulates the ligand-induced transactivation function of these receptors, although it is devoid of intrinsic transcriptional activity when tethered to DNA. In addition, the ectopic coexpression of vinexin alpha and ERalpha results in a loss of ERalpha phosphorylation on serines and the partial redistribution of vinexin alpha into the nucleus, where it colocalizes with ERalpha. These results establish a new model of transcriptional regulation where components of the cell-cell and cell-substrate adhesion complexes can regulate the phosphorylation and activity of steroid receptors.

Vinexin, CAP/ponsin, ArgBP2: a novel adaptor protein family regulating cytoskeletal organization and signal transduction

Adaptor proteins, composed of two or more protein-protein interacting modules without enzymatic activity, regulate various cellular functions. Vinexin, CAP/ponsin, and ArgBP2 constitute a novel adaptor protein family. They have a novel conserved region homologous to the active peptide sorbin, as well as three SH3 (src homology 3) domains. A number of proteins binding to this adaptor family have been identified. There is accumulating evidence that this protein family regulates cell adhesion, cytoskeletal organization, and growth factor signaling. This review will summarize the structure and the function of proteins in this family.